Injection drug use increases the probability of contracting HIV, and opioids accelerate the progression of HIV-1 infection through immune suppression and direct CNS actions. Opioid receptors are widely expressed on immune cells and glia, and opioids can modulate chemokine signaling. CCR5 plays a critical role in HIV infection as a co-receptor for entry of CCR5-preferring strains. CCR5 levels in general, and in microglia/CNS macrophages, have been correlated with severity of HIV neurologic disease. Individuals homozygous for the CCR5?32 mutation resist infection, and the CCR5 antagonist maraviroc (MVC) is used clinically to inhibit HIV entry. Results from the last funding period using non-infective paradigms reveal that CCR5 may also promote HIV-related neuropathology apart from its role in enhancing HIV infection. For example, blockade of CCR5 with MVC increases synaptic plasticity, reverses Tat-induced reductions in antinocicetion, tolerance, and morphine dependence, and enhances survival of Tat-treated striatal neurons. CCR5 overactivation may be a pivotal factor in the ability of opiates to amplify HIV neuropathogenesis, and heterologous interactions between MOR and CCR5 likely underlie many CNS deficits in HAND. CCR5 and MOR interactions in the context of HIV are likely fundamentally different among CNS regions due to distinct glial/neuronal distributions of CCR5 and MOR, their ligands, and effector coupling. Critical to this proposal, MVC rescued Tat-induced deficits in a spatial learning/memory task involving hippocampal function (Barnes maze), and hippocampal damage is involved in learning/memory deficits in HAND patients, irrespective of cART. To explore the hypothesis that aberrant CCR5-MOR interactions underlie HIV-induced hippocampal deficits, we will systematically interrogate those interactions in the context of HIV/Tat and opiate exposure. In vivo and ex vivo studies in HIV-1 Tat transgenic mice are complemented by in vitro studies using human neurons exposed to Tat, gp120, and HIV. Three interrelated Aims test hypotheses in male and female mice. Aim 1 tests CCR5 impact on the dynamics of HIV-1/Tat and MOR interactions in the hippocampus, using functional assessments of behavior (spatial and non-spatial tasks) and neurophysiology (LTP and membrane properties in ex vivo slices). Aim 2 tests how CCR5 activation or blockade (genetic, MVC) alters HIV-1/Tat and MOR interactions, using Tat, gp120, and HIV in murine or human co-cultures of hippocampal neurons and glia. Neuron dysfunction (optical physiology), [Ca2+]i homeostasis, synaptodendritic injury, and death, as well as underlying inflammation and signaling pathways involved (bioplex/MSD multiplex) are explored. Aim 3 examines heterologous interactions between MOR and CCR5, assessing receptor dynamics and activity by [3H]naloxone or [3H]maraviroc-stimulated [35S]GTP?S autoradiography in hippocampal sections, and concentration-effect binding curves in membranes from brain and cultured neurons/glia. TrkB and p75NTR roles in regulating outcomes tested pharmacologically; protection by altering the proBDNF/mBDNF balance (tPA to increase proBDNF cleavage) assessed in all Aims.